1. Field of the Invention
The invention relates to CATV systems, and more particularly to a method and apparatus for selectively jamming selected channels to selected subscribers in a CATV system.
2. Description of Related Art
CATV systems in which there are a number of channels provided to the subscribers are well known. Many CATV systems charge different amounts to their various subscribers, depending on how many of the supplied channels the particular subscriber wants to receive. For example only, a CATV system may offer channels 1 through 12 inclusive. The charge for viewing channels 1 through 9 inclusive might be $Y.00 per month. The charge for viewing channels 1 through 9 inclusive plus also channel 10 might be $Y.00+$10.00 per month. The charge for viewing channels 1 though 9 inclusive plus channels 11 and 12 might be $Y.00+$20.00 per month. Accordingly, since all of the subscribers are connected to the same controlled access medium a means has to be found to selectively deny channels 10 and/or 11 and/or 12 to those subscribers who have not paid the additional charge(s).
It is known in the art that certain channels can be scrambled prior to transmission from the supplier's head end and subsequently descrambled at the home ends of those subscribers who have paid for those scrambled channels. A device commonly referred to as a descrambler is installed in the paying subscribers homes, which restores the scrambled television signal. One problem with such a system is that the descrambler must be installed, thus access to the subscriber's home and manpower is needed. A second problem occurs if the subscriber no longer wants the channels, or defaults in paying for the channels, again, access to his home and manpower is needed to remove the descrambler. A third problem is that if a subscriber can buy or build an illegal descrambler he can obtain the additional channels without paying for them.
The other known means of selectively denying channels to selective subscribers is by selectively jamming the channels to be denied to the subscribers who have not paid for them. One known method of jamming channels is to send, along with the signal of each channel to be jammed, a signal having a repetition rate close to the vertical syncronization rate and of a greater amplitude than the vertical synchronization signal. The television will pick up the jamming signal as the synchronization signal and will therefore produce a vertically unstable picture. One problem with this means is that a vertically unstable picture is not as unwatchable as a horizontally unstable picture.
Another known means of jamming channels is to send, along with the signal of each channel to be jammed, a signal having a repetition rate close to the horizontal synchronization signal and of an amplitude greater than the horizontal synchronization signal. The television will pick up the jamming signal as the synchronization signal and will therefore produce a horizontally unstable picture.
A common problem with both of the known methods and ways in which they are known to be carried out is that the interfering signal must be produced, and it must be combined only with the television channel signal going to the subscribers who have not paid for that channel. If the interfering signal were combined with the television signal to all subscribers they would all have that channel jammed.
A known means of selectively combining each of the different jamming signals that are being put out for each of the different channels to be jammed with only those television signals going to the subscribers who are not authorized for that particular channel is used in U.S. Pat. No. 4,521,809. The known means used in U.S. Pat. No. 4,521,809 requires one separate line per subscriber and one switch per subscriber per channel that might be jammed. The separate subscriber lines are each coupled to a different switch for each channel that might be jammed, thereby creating a switch and subscriber line matrix in which each subscriber line has a unique switch for each channel that might be jammed, controlling that subscriber's access to said channel, and in which all switches controlling subscriber access to a particular channel are connected to each other. For example only, if the situation is that channel "A" is to be jammed to subscribers "1", "2", and "3", each of the channel "A" switches that couple the jamming signal means to the subscriber lines " 1", "2" and "3" are closed. Accordingly, if there are 1000 subscribers and 5 channels to be jammed, 1000 separate subscriber lines are needed, and (5.times.1000)=5000 switches are required. Accordingly, a large number of switches is required. Additionally, it would not be an easy matter to add further subscriber lines and switches to the switch and subscriber line matrix.